Operators of fleets of rental moving vans powered by diesel engines have encountered problems with renters refilling fuel tanks with gasoline rather than diesel fuel. Most often the person who rents the vehicle is totally unfamiliar with the vehicle and its propulsion system. Force of habit formed through experience with his own personal passenger vehicle leads him to pump gasoline. And the distractions associated with the operation of a vehicle with which he is not familiar may cause him to fail to heed simple warning legends, such as "Diesel Fuel Only" on labels placed near the tank filler tube.
In the wholly unrelated field of containers for poisonous substances it has been proposed to associate signal flags with a bottle stopper to alert the user as to the nature of the contents when the stopper is removed. See U.S. Pat. No. 1,008,647 granted Nov. 14, 1911, to O. M. Kelso for "POISON BOTTLE". So far as is known no such alerting system has ever been adapted to vehicle fuel tank closure caps.